The present invention relates to a brake booster for an automotive vehicle, which utilizes the pressure differential between a vacuum and atmospheric pressure and comprises a vacuum casing having at least one movable wall disposed therein, the movable wall dividing the vacuum casing into a vacuum chamber and a working chamber and acting upon a push rod guided in the axial direction of the brake booster.
For German Patent DE-AS No. 2,345,314, a brake booster for an automotive vehicle is known which is operated by the differential of pressure between a vacuum and atmospheric pressure. The brake booster comprises a vacuum casing, a movable wall disposed therein and a control valve, the movable wall dividing the vacuum casing into a vacuum chamber of constant vacuum and a working chamber of different pressures by means of a diaphragm and acting on a push rod guided in the axial direction of the brake booster. The transverse end wall of the vacuum casing adjacent the control valve includes several bolts serving to secure the brake booster to the splash wall of an automotive vehicle. The transverse end wall of the vacuum casing adjacent the end of the push rod which extends out of the vacuum casing is employed to mount the master cylinder to the vacuum casing. As a rule, this mounting is accomplished by means of bolts bolted to the master-cylinder flange.
In an arrangement of this type, the vacuum casing is conventionally made of sheet steel. As a result of the pressure differential between vacuum and atmospheric pressure, pneumatic forces act on the vacuum casing. These forces, which somewhat compress the vacuum casing, have to be taken up by the components of the vacuum casing. To limit the amount of deformation of the vacuum casing, it is necessary to provide the casing components with an appropriate strength. These designs are, however, not economical because of the resultant increase in material costs in addition to being of high weight. Thus, such an arrangement is unable to comply with the vehicle manufacturers' demands for a lowest possible weight of brake systems without the safety and operability of the brake booster being impaired. cl SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a brake booster of the type described hereinabove in which the material costs and the weight are reduced.
A feature of the present invention is the provision of a brake booster for an automotive vehicle, which utilizes the pressure differential between a vacuum and atmospheric pressure comprising:
a vacuum casing having a longitudinal axis and at least one movable wall disposed therein, the movable wall acting upon a push rod movable along the axis and dividing the vacuum casing into a vacuum chamber and a working chamber; and PA1 a pipe disposed within the vacuum casing approximately coaxial of the axis and fastened to opposite transverse end walls of the vacuum casing, the pipe enclosing the push rod, the movable wall being sealed to the outer surface of the pipe and slidable thereon. PA1 (a) the lost travel of the brake pedal is reduced substantially because the vacuum casing is not subjected to the actuating force; and PA1 (b) the negative effects, with respect to the lost travel, of the pneumatic forces acting on the vacuum casing are eliminated.
The essential advantages of the brake booster of the present invention are particularly that the weight of the brake booster is reduced by the elimination of reinforcements and the use of thinner-walled components, and that it is possible to manufacture the vacuum casing using lower-weight and less costly materials.
The arrangement disclosed in the present invention is of particularly importance for a mechanically controlled brake booster utilizing the pressure differential between a vacuum and atmospheric pressure and comprising a vacuum casing having at least one movable wall disposed therein, and a pressure control valve, in which the movable wall divides the vacuum casing into a chamber and a working chamber and acts upon a push rod guided in an axial direction of the brake booster, and in which the pressure control valve includes a control casing which is located at least partially within the vacuum casing with the vacuum casing being fastened to a splash wall by its one transverse end wall and a master cylinder is fastened to its other transverse end wall. The vacuum casing of such a brake booster as it is described, for example, in the above-mentioned German Patent is subjected to high forces, and this is the main problem to be overcome by the present invention.
As the brake pedal is depressed, the force transmitted by the brake pedal and the booster force act on the push rod. The resultant total force acts on the piston of the master cylinder, with the resistance of the compressed hydraulic fluid in the master cylinder necessitating, however, the presence of an opposed force component maintaining the master cylinder approximately in its position. This reaction force is transferred from the master cylinder via the sheetmetal components of the vacuum casing to the splashwall to which the pedal support is usually fastened.
On actuation of the brake booster, the reaction force results in an elongation of the vacuum casing in an axial direction and, consequently, in a displacement of the master cylinder which adds to the lost travel of the brake pedal. Because of the transmission ratio of the brake pedal, the elongation of the vacuum casing is increased bu a mutiple and transmitted to the brake pedal. The elongation is still further increased by the pneumatic forces acting on the vacuum casing. Their effects have to be taken into consideration to a still greater extent if the casing components are of reduced wall thickness.
Therefore, in mechanically controlled brake boosters, the advantages of the arrangement of the present invention, in addition to those mentioned in the foregoing, are in particular that
It is an advantage that the control casing is slidably arranged in the pipe in the manner of a piston and sealed relative to the pipe by means of seals, one seal being located on the control casing's end adjacent the push rod and the other seal close to the pipe's end adjacent the splash wall. This arrangement enables the control valve casing to be situtated within the pipe whereby a space-saving construction is achieved.
In a preferred embodiment of the subject matter of the present invention, the pipe includes at least two slots extending in a longitudinal direction and receiving slidably therein at least one cross member to mechanically couple the movable wall to the push rod. The length of the slots determines the maximum displacement travel of the movable wall and the push rod, respectively. In an arrangement which is simple with respect to assembly and safe with respect to the mechanical load, the cross member engages a groove disposed on the circumferential surface of the control casing or the push rod normal to the longitudinal axis and is rigidly secured to the movable wall.
The movable wall includes advantageously a hub extending along the entire length of the slots, with the cross member being secured to the one end of the hub while its other end is provided with a slide seal. The movable wall is, thereby, at a small distance from the pipe along the length of the slots and, accordingly, the cross members may be of small height. The cross member is suitably of U- or T-shaped design. Two T-shaped or, alternatively, one U-shaped cross member may be used.
In an advantageous arrangement, two movable walls and a rigid partition wall arrangement between the two movable walls and fastened to vacuum casing are provided, and the movable walls are rigidly coupled to each other by means of two rods extending in parallel with the push rod. By these means, the two movable walls are moved synchronously, and the force acting thereon is transmitted to the push rod without the necessity of providing additional slots in the pipe. The rods extend through openings in the rigid partition wall, which openings are provided with slide seals. The working chambers are connected in a particularly advantageous manner requiring no additional elements by designing the rods as tubes, with an open end of each tube extending into the outward working chamber, and by providing radial openings close to the other tube end.
To seal the inward edge of the rigid partition wall on the pipe's circumferential surface, the inward edge of a rolling diaphragm of the movable wall is suitably clamped between the partition wall and the pipe. This arrangement obviates the necessity for a separate seal. To enable the pipe to be used as a connecting channel for the vacuum chambers, the pipe includes advantageously openings in the area of the vacuum chambers.